For many years, two principal methods were common for the production of flexible hoses made by soft thermoplastic material reinforced with a continuous spiral made by a rigid thermoplastic material. The materials which are used for these hoses are mainly the combination of soft (plasticized) and rigid PVC. The calibration of the soft PVC at the specified dimension by the usual methods is very difficult because this material behaves as a viscous glue at the processing temperatures. On the other hand this property of the plasticized PVC, which under the normal environment conditions behaves generally as a rubber, makes it incomparable for the creation of flexible hoses reinforced with a strengthening spiral from hard thermoplastic or metallic spiral, which is embodied in its mass consisting of the wall of the hose. This type of hose keeps their cylindrical form and presents a satisfactory resistance to the external and internal loads. Because of their flexibility they are very useful and almost non-replaceable for many applications such as the transport of water, petrol, beer, milk and many liquids, for the supply or absorption of air, absorption of industrial gases, powders etc., and further for transportation of gravel, dry seeds etc., protection and additional external isolation of electric cables any many other special applications.
According to the first method of production, which was invented by Arist. Petzetakis, see Greek patent No 22347/13th of Oct., 1961, a simultaneous coextrusion of the two materials, soft and rigid PVC is made, with the soft material surrounding the rigid material. Through a special head, the produced composite "cord" is wound around a special calibration unit, which gives the produced hose the desired diameter. As this "cord", which is composed by the two melted plastic materials, comes in contact with the wall of the already formed plastic hose, it sticks on it, forming an endless spiral. As indicated above, the adhesive soft material surrounds the corresponding rigid portion, so actually, the soft material is the one that comes in continuous contact with the corresponding soft material of the last spiral of the already formed hose and, by continuous welding, forms the hose. In this way the advantageous weldability of the soft PVC and other soft plastic material is completely exploited. However, because of the above required property, the melted soft PVC also sticks easily on the material of the necessary calibration unit, thereby not permitting the creation of a hose with satisfactory quality. To avoid this problem, a special calibration unit is used in connection with this method wherein the produced hose while it is pulled away with the help of a haul-off machine, laying post the production line.
By this method the hose is produced linearly (does not rotate during production), the required area is small and there is no limitation to the produced length. However, the hoses produced by this method have a serious disadvantage relating to the quality of their internal surface. Anybody can easily observe the deep longitudinal grooves, known as tape marks, on their entire internal surface, which sometimes wound even the reinforcing rigid plastic spiral.
These internal grooves constitute lines of inferior strength, when subjected to internal or external loads. Also, they reduce the impact strength of these hoses especially at low temperatures. Furthermore, these grooves attract the inhabitance and the development of parasites, bacteria and various microbes, which restricts their usage for foodstuff applications.
According to the second production method, which was developed by the japanese SHIRO-KANAO, patent No SHO 35-7629, the two plastic materials, soft and rigid, are also coextruded with the soft material surrounding the rigid one. The produced composite "cord" is led tangentially to a special calibration unit, which consists of a series of peripheral rollers, which are held as a cantilever on an immobile basement. These rollers are disposed at an inclination to the central axis of the calibration unit and can be rotated with the desired speed via motor and a gearbox.
This roller rotation is transmitted to the tangentially fed melted plastic "cord" which is thus pushed towards the exit of the calibration unit. The contact of the soft sticky plastic with the rotating rollers is instantaneous, as they are rolling under it, and its adherence to them is avoided. At the same time, the external surface of the melted soft plastic comes in contact with the corresponding, still in melted stage, lateral surface of the already formed hose, which is still on the calibration unit, sticks to it and forms the new spiral of the produced flexible hose.
As it is easily perceptible, hose produced by this method rotates during its production around its axis, which at the same time is the central axis of the calibration unit. This rotation is transmitted from the rotating peripheral roller to its internal surface as it comes successively in contact to each one of them and the produced hose cools down, shrinks and is tightened on them.
With this method, a successful production of flexible plastic hoses is also achieved. Internally, the produced hoses have a smooth surface and they do not have the detrimental longitudinal grooves of the preceding method. However the disadvantage of this method is the obligation to continuously rotate the flexible plastic hose during its production. This problem becomes more apparent a the hose diameter is smaller. The continuous trouble-free rotation of, for example, three hundred feet of a flexible plastic hose, specially while this rotary motion is exerted to its one and which is still hot, is not a stable process. Any slight disturbance would cause problems at the hose generation area. Furthermore the total floor length required for such a production line is big and relative to the needed length of the hoses. The required marking on the hose's surface is not possible to be done at the same time as the hose is produced due to its rotation. Neither is its simultaneous winding in coils possible. These procedures have to be executed in a second stage, which affects the production cost accordingly.
Less used is a variation of the second method of production. According to this method, ACHENBACH, the produced coaxial plastic "cord" is initially cooled-down and then is fed tangentially to a relatively short rotating conical calibration unit. There, the lateral sides of the "cord" and the already formed hose are heated intensively to melt, with the aim of hot air and, immediately after they are fused together with the additional help of local compression.
This method, like the preceding one, has the disadvantage of the obligatory hose rotation during its production. Beyond this, the hoses produced by this method hoses contain internal stresses in their wall, due to the cold winding of the plastic "cord". These stresses negatively affect the hose's properties.